NMR or MRI
In MRI systems or nuclear magnetic resonance (NMR) systems, a static magnetic field B.sub.o is applied to the body under investigation to define an equilibrium axis of magnetic alignment in the region of the body under examination. An RF field is then applied in the region being examined in a direction orthogonal to the static field direction, to excite magnetic resonance in the region, and resulting RF signals are detected and processed. Generally, the resulting RF signals are detected by RF coil arrangements placed close to the body. See for example, U.S. Pat. No. 4,411,270 to Damadian and U.S. Pat. No. 4,793,356 to Misic et al. Typically, such RF coils are either surface type coils or volume type coils, depending on the particular application. Normally separate RF coils are used for excitation and detection, but the same coil or array of coils may be used for both purposes. For multiple surface RF coils for use in NMR, see U.S. Pat. No. 4,825,162 to Roemer, et al.
A further increase in S/N can be realized with the use of quadrature coils as compared to the conventional linear coil designs. See for example U.S. Pat. No. 4,467,282 to Siebold and U.S. Pat. No. 4,707,664 to Fehn. Also see U.S. Pat. Nos. 4,783,641 and 4,692,705 to Hayes for a quadrature volume coil, commonly referred to as the "birdcage" coil in the NMR community. For the use of multiple volume coils for use in NMR, see U.S. Pat. No. 5,258,717 to Misic, et al., and the reference article by Leussler for the use of multiple volume coils for simultaneous head and neck imaging (See, C. Leussler, "Optimized Birdcage Resonators for Simultaneous MRI of Head and Neck", SMRM 12th Annual Meeting, New York, Book of Abstracts, page 1349, 1993). Also, reference is made to commonly assigned U.S. patent application Ser. No. 08/745,893 filed on Nov. 8, 1996 titled "Radio-Frequency Coil and Method for Resonance Imaging/Analysis", and Ser. No. 08/993,932 entitled "Improved Radio-Frequency Coil and Method for Resonance Imaging/Analysis", filed on Dec. 18, 1997, the disclosures of which are incorporated herein by reference, for the use of multiple volume and surface coils for use in NMR imaging.
The recent introduction of array coils to NMR, has led to commercially available cervical-thoracic-lumbar (CTL) array coil for entire spine imaging, and flexible body arrays for torso imaging. These multichannel coils significantly help reduce scan times. A routine MR study takes approximately 45 minutes, including the patient placement. This is uncomfortable especially for claustrophobic patients in general. In addition, prolonged scan times make the contrast-enhanced studies even more difficult to obtain. The almost 1 hour MR study with and without the contrast agent makes MR not so suitable for imaging emergency trauma cases.
This necessitates a new array coil with high S/N, that will allow the MR study of the torso, head, spine or joints such as the knee, wrist and shoulder etc., to be performed in reduced scan times. This will significantly reduce patient discomfort and increase patient throughput in a MR scanner. The reduced scan times will also allow MRI systems to be used in scanning emergency trauma patients.
A new area of MRI namely functional MRI or more commonly referred to as fMRI has emerged in the recent years. This technique provides the capability of mapping the brain functions, non-invasively using MR. Unfortunately, a major drawback of this technique is its lack of sensitivity. Once again, a coil with improved S/N will provide a much clear image that will assist in the diagnosis of disorders in the human brain.
Nuclear Quadrupole Resonance (NQR)
NQR is a technique that is capable of locating and uniquely identifying nitrogen for the detection of explosives and/or narcotics, even when contained and concealed by other materials. NQR has potential application in general and medical imaging and industrial measurements, in addition to the detection of either explosives (including land mines) or narcotics. See U.S. Pat. Nos. 5,594,338 and 5,592,083 for the design of an RF coil employed in the NQR system.
Generally, a significant factor in contraband detection by means of NQR is that quadrapolar nuclei that are commonly present, and potentially readily observable, in narcotics and explosives include nitrogen (.sup.14 N) and chlorine (.sup.35 Cl and .sup.37 Cl), among possible other nuclei. Thus, in commercial applications it is necessary to be able to detect quadrapolar nuclei contained within articles of mail, mail bags or airline baggage, including carry-on and checked luggage. The resonant frequencies of the nitrogen and chlorine in these substances are different for each chemical structure, but are well defined and remain consistent. That is, for a given chemical structure the resonance frequencies for nitrogen and chlorine compounds remain intact and do not change, unless their chemical structure is altered.
Generally, NQR frequencies of quadrapole nuclei lie within 0.5-5 MHz range. However, for organic chlorine compounds, .sup.35 C1 chemical shifts range from 16-55 MHz. The chemical shift of chlorinated hydrocarbons occurs between 32-45 MHz. This is a very wide frequency range for one single turn RF coil of the aforementioned '338 and '083 references to cover.
Even the 0.5 to 5 MHz (a ten fold frequency) range of detection for .sup.14 N in explosives and or narcotics mandate a capacitance of a factor of 100 (f.sup.2 .varies.1/LC) to tune the coil from 5 to 0.5 MHz range, which are overwhelmingly large range of capacitances required to tune the RF coil. Since, the same RF coil was used for a wide frequency range, the RF coil design was un-optimized for the several frequency ranges of operation. This may affect the performance of the RF coil (Q values) and the entire NQR system (transmitter power, S/N), in the detection of low levels of nitrogen compounds found in plastic explosives and narcotics.
This necessitates that the RF coil design be optimized for maximum S/N over at least a majority of the frequency ranges of NQR operation and detection in reduced examination times.
Distributed Type Volume Coils
Birdcage Coil
Even after several years following the introduction of array coils to NMR, the only coil that is commercially used for scanning the human head in a horizontally oriented B.sub.o magnetic field is the quadrature birdcage coil of Hayes '705. Other applications of this coil design are for the whole body, knee and wrist imaging. A birdcage coil consists of two rings connected by several straight segments referred to as legs. A planar schematic of an eight leg high-pass birdcage is shown in FIG. 1a. This coil consists of two end rings R1 and R2 and 8 legs 1 through 8. Each ring section between two legs are interrupted by two series 2C value capacitors. Their combined effect is one capacitor of C value. FIG. 1b is the front view of the birdcage describing the location of the ring with respect to the legs and includes the mode orientation. FIG. 1c is the side view of the coil outline shown for brevity.
The birdcage which is of the distributed inductance-capacitance type structure has several frequency modes. Of interest is the first or principal or k=1 quadrature mode. This k=1 quadrature mode has two linear components (1a, 1b), oriented orthogonal to one another as shown in FIG. 1b. As mentioned above, the quadrature coil provided a 41% improvement over the conventional linear coil designs. The birdcage expended half the power when compared to the conventional linear coil, thus significantly reduced the RF power deposited in the patient. The higher order or k&gt;1 modes had no net field at the coil center and generally were not used for imaging. At the k=1 mode, the currents in the coil were cosinusoidally distributed such that the resultant field displayed a homogeneous distribution over the imaging field-of-view (FOV). It is for these regions this coil gained popularity in the NMR community for the several volumetric applications (torso, head, knee, wrist, etc.).
The dashed lines of FIGS. 1a, 1b and 1c are planes of symmetry for this birdcage. From FIG. 1b, there are four such planes (I, II, III, IV), that are distributed azimuthally (due to symmetry). There is one additional axial plane (V) that is centrally located between the two end rings R1 and R2, dissecting the coil axis (see FIG. 1c) which, in addition is also a virtual ground plane. The points where the planes of symmetry intersect the birdcage are "a, b, c, d, e, f, g, h" on ring R1 and "i, j, k, I, m, n, o, p" on ring R2, "q, r, s, t, u, v, w, x" on legs 1, 2, 3 . . . 8 respectively of FIG. 1a. Since points "q-x" are located on the virtual ground plane, these points are at virtual ground potential or have no net potential.
Should points "a-p" on the end rings be connected as shown in FIG. 1d, then the 8 leg coil of FIG. 1a will become a 16 leg coil of FIG. 1d and the frequency mode structure including the current distribution will be altered. The resultant structure in this case was still a single birdcage, even after the addition of eight more legs. Thus the increase in S/N was not realized even after this addition, although the homogeneity along the axial planes of the coil may have improved slightly over the eight leg coil. And since no increase in S/N was realized, this approach was unacceptable.
However, should the virtual ground points "q-x" in the legs of FIG. 1a be shorted, this will result in the coil of FIG. 1e. This will give rise to a new RF gradient mode, bi-phasic in nature with + & - lobes along the coil axis. However, it is noted that a RF gradient mode for the coil of FIG. 1e, has no net field at the coil center (i.e., the RF gradient mode has no net field in the central virtual ground plane of FIG. 1c). Therefore, although FIG. 1e has two birdcages that share one end ring R.sub.12 and even a new mode is realized, no net increase in S/N at the coil center is realized.
3-Channel Distributed Type Coil Head Array
A quadrature, 3-channel head coil was described by the inventor in previously filed Ser. No. 08/993,932, which provided improved S/N at the coil center and toward the top of the head (see FIG. 2). The coil consisted of two birdcages (coils #1, #2), one distributed, quadrature modified surface coil (coil #3) and passive circuits were used for decoupling individual coils and to minimize the cross-talk between all coils in the array. The coil was operated in the multiple operating modes, with focus to the upper or lower portions of the brain or for routine head studies in one clinical setting, with high S/N and without compromising homogeneity. Here the birdcage, coil #2 and the quadrature surface coil #3 were asymmetrically overlapped and therefore isolated from one another and is the subject of previously filed U.S. Ser. No. 08/745,893.
The combination of coils #2 and #3 was then overlapped with birdcage coil #1. Since all three coils in the array were physically separated from one another, and were overlapped to maintain minimal mutual coupling, each coil in the array maintained their own RF current distribution and mode orientation. Several passive coil-to-coil decoupling electronics helped minimize the residual cross-talk between coils in the array. Each quadrature coil signal was routed to individual low noise figure, high gain preamplifiers before digitization. Diode protection circuits were inserted between the coil and the respective preamplifier for preamplifier protection during whole body transmit.
2-Channel Birdcage, Head and Neck Array
A quadrature, 2-channel birdcage array was described in C. Leussler, "Optimized Birdcage Resonators for Simultaneous MRI of Head and Neck", SMRM 12th Annual Meeting, New York, Book of Abstracts, page 1349, 1993 for simultaneous head and neck imaging (see FIG. 3). This coil involved 2 birdcages; a coil 10 for the head and a coil 12 for the neck. The head birdcage 10 had eight fold symmetry, whereas the neck birdcage 12 had only a four-fold symmetry. The neck birdcage 12 had shoulder cut-outs for accomodating the neck as shown in FIG. 3. This coil provided an extended FOV without significantly compromising S/N and homogeneity over the extended FOV. Nevertheless, no increase in S/N was realized over extended FOVs. That is, the S/N of the array coil was comparable to individual head or neck coils over the head & neck scan volume.
Distributed Type Surface Coils
The distributed surface coil of FIG. 4a has three meshes, 4 vertical segments referred to as legs and 2 horizontal segments referred to as ring segments. Each of the ring segments between the legs are broken with two 2C capacitors in series. Like wise, each of the end ring segments are also populated with two 2C value capacitors. For details of this coil design, refer to U.S. Pat. No. 4,783,641 to Hayes, et al.
The coil of FIG. 4a is of the high-pass configuration, and has three resonance modes. The principal or k=1 mode behaves like a simple loop coil of identical outside coil dimension of FIG. 4a (see FIG. 4b). The k=2 mode will behave similar to a butterfly design of FIG. 4c. Dashed lines I, II and III merely depict the mid voltage points "a-f" between 2 capacitors of identical values, whereas line IV is a virtual ground plane and points "g-j" are at virtual ground potential or have no net potential.
The main advantages of the distributed type coil designs are its distributed sinusoidal currents that help provide uniform B field distribution, and reduced losses. Also, the field profiles lay close to the coil which minimize tissue losses from the human body, thus increases coil S/N.
A multiple surface coil arrangement disclosed in U.S. Pat. No. 5,256,971 to Boskamp is shown in FIG. 4d. Here, two surface coils of similar dimension are overlapped for minimum mutual inductance from one another. A third coil is added to this set, such that the third coil is magnetically isolated from the first and second coils. Here, all three coils are mutually isolated from another. In doing so, the third coil has a different coil geometry than the first and second coils, and extends beyond the FOV of the first and second coils combined.
Should this arrangement of coils be flexed around the human torso, the isolation between the third coil and the first and second coils will change, which in turn will affect the isolation between the first and second coils, as the first and second coils will now start to couple via the third coil. That is, should the third coil begin to couple to either the first or second coils, all coils in the array will begin to couple with each other. This was not satisfactory.
This necessitates a coil system where the individual coils in the system are well isolated from one another and still maintain its current distribution and preferred mode orientation irrespective of its shape.
Single and Multiple Turn Solenoid Type Coils
Solenoid Coil for NMR
One of the oldest and perhaps the most popular coil design that is commercially utilized for the several volumetric applications (torso, head, spine, knee, wrist) is of the solenoid design. See for example, U.S. Pat. No. 4,398,148 to Barjhoux et al.
FIG. 5a is one example of a solenoid head coil configuration commonly used in the NMR community. The N-turn solenoid is resonated with two series connected 2C value capacitors. This coil has 2 planes of symmetry, I and II, respectively. Plane I intersects the coil at virtual ground points "a , b". A side view of the coil outline along with the head and the central virtual ground plane I is shown in FIG. 5b, for brevity.
Shorting the two virtual ground points of FIG. 5a will result in FIG. 5c. This will give rise to a new RF gradient mode along the coil axis. It will be noted that a RF gradient mode has no net field at the coil center (i.e. the RF gradient mode has no net field in the central virtual ground plane of FIG. 5b). Therefore, although FIG. 4c has two solenoid coils sharing the two virtual ground point "a, b" of FIG. 5a and even a new gradient mode is realized, the homogeneous mode of FIG. 5a will not be affected and no net increase in S/N is realized at the coil center.
Single Turn Solenoid for NQR
A single turn solenoid coil of FIG. 6 was used to detect the .sup.14 N signals in crystalline form for detecting concealed explosives and narcotics employing nuclear quadrupole resonance (NQR). See U.S. Pat. Nos. 5,594,338 and 5,592,083 for the design of an RF coil employed in the NQR system.
FIG. 6 has one single turn RF coil which is tuned to a wide range (approx 0.5 to 5 MHz), by simply adding large and small value capacitances for coarse and fine tuning with the help of relay switches. As seen, the upper frequency range was ten fold of the lower range which mandated a 100 fold change in capacitance to tuned the coil. Since, the same RF coil was used for a wide frequency range, the RF coil design was un-optimized for the several frequency ranges of operation. This may affect the performance (transmitter power, S/N) of the RF coil and the entire NQR system, in the detection of low levels of nitrogen and chlorine compounds found in plastic explosives and narcotics.
This necessitates that the RF coil design be optimized for at least a majority of the frequency ranges of NQR operation and detection which will also help in reducing examination times.
This RF coil design will allow for at least one optimized coil in the array that will cover a part of the frequency spectrum, such that all coils in the array combined cover the entire frequency spectrum required for detection. This will help reduce the overall scan frequency range per coil and thus allow rapid tuning of coils in the array. This RF coil design may also be designed to allow for multiple tuning of the coils in the array without crosstalk and capable of simultaneous operation, which will help scan the entire frequency range in reduced scan times.
It is therefore a primary objective of the present invention to further improve S/N and reduce scan times of all such coil systems used for resonance imaging or spectroscopic analysis mentioned above. Specific applications of the coil described herein in accordance with the present invention include distributed type surface and volume coils, and single and multiple turn solenoid type coils.